Sequence Stratigraphy Concepts

Contents

Introduction Sedimentology concepts Fluvial environments Deltaic environments Coastal environments Offshore marine environments

Sea-level change Sequence stratigraphy

concepts Marine sequence stratigraphy Nonmarine sequence

stratigraphy Basin and reservoir modeling Reflection

Sequence stratigraphy concepts

Sequence stratigraphy highlights the role of allogenic controls on patterns of deposition, as opposed to autogenic controls thatoperate within depositional environments Eustasy (sea level) Subsidence (basin tectonics) Sediment supply (climate and hinterland tectonics)

Surface processes reflecting dynamic interplay of sediment supply and wind energy in eolian environments.

Sediment supply exceeding the transport capacity (energy) of winds results in the accumulation of sand as sheets or dunes.

Winds stronger relative to their sediment load lead to erosion and the formation of deflation surfaces.

MARS ROVERNAMIB DESERT

Arabian peninsula


Accommodation is the space available, at any given point in time, for sediments to accumulate; accommodation is created or destroyed by RSL changes

Water depth is controlled by changes in accommodation as well as sedimentation

Base level is the horizontal surface to which subaerial erosion proceeds; therefore it corresponds to sea level

Base level is a principal control of accommodation, and, hence, whether erosion or deposition is likely to occur at any given location; attempts to extend the concept landward are controversial


Allostratigraphy is a relatively new approach to stratigraphicsubdivision, and is based on the separation of strata based on unconformities or other discontinuities (e.g., paleosols)

Sequence stratigraphy is the analysis of genetically related depositional units bounded by unconformities and their correlative conformities

A depositional sequence is a stratigraphic unit bounded at its top and base by unconformities or their correlative conformities(=allostratigraphic unit), and typically embodies a continuum of depositional environments, from updip (continental) to downdip(deep marine)

The subtle balance between RSL and sediment supply controls whether aggradation, regression (progradation), forced regression, or transgression (retrogradation) will occur

Transition from marine to nonmarine environments. Large arrows:direction of shoreline shift in the two river mouth environments (R regressive; T transgressive).

Between the river mouth environments, the coastline is an open shoreline. Note that the character of the shoreline (transgressive vs. regressive) may change along strike due tovariations in subsidence rates and sediment supply.

Gilbert-type delta front, prograding to

the left (Panther Tongue, Utah). The

delta front clinoforms downlap

the paleoseafloor(arrows)

River-dominated delta showing prodelta fine-grained facies at the base, delta frontsands prograding to the left, and coal-bearing delta plain facies at the top (the Ferron

Sandstone, Utah). The prograding delta front clinoforms dip at an angle of 5-7, and downlap the underlying prodelta deposits (arrows).

2D seismic transect showing progradation of a divergent continental margin. The shelf edge position can easily be mapped for consecutive time slices

The prograding clinoforms downlap the seafloor (yellow arrows), but due to the rise of a salt diapir (blue arrow) some downlap type of stratal terminations may be confused with

onlap (red arrows)

Log motifs of a low-energy fluvial system, showing both fining-upward (channel fills CH) and coarsening-upward (crevasse splays CS) trends.


Allostratigraphy is a relatively new approach to stratigraphicsubdivision, and is based on the separation of strata based on unconformities or other discontinuities (e.g., paleosols)

Sequence stratigraphy is the analysis of genetically related depositional units bounded by unconformities and their correlative conformities

A depositional sequence is a stratigraphic unit bounded at its top and base by unconformities or their correlative conformities(=allostratigraphic unit), and typically embodies a continuum of depositional environments, from updip (continental) to downdip(deep marine)

The subtle balance between RSL and sediment supply controls whether aggradation, regression (progradation), forced regression, or transgression (retrogradation) will occur


A RSL fall on the order of tens of meters or more will lead to abasinward shift of the shoreline and an associated basinwardshift of depositional environments; commonly (but not always) this will be accompanied by subaerial exposure, erosion, and the formation of a widespread unconformity known as a sequence boundary

Sequence boundaries are the key stratigraphic surfaces (high-order bounding surfaces) that separate successive sequences and are characterized by subaerial exposure/erosion, a basinward shift in facies, a downward shift in coastal onlap, and onlap of overlying strata

Parasequences are lower order stratal units separated by (marine) flooding surfaces; they are commonly autogenic and not necessarily the result of smaller-scale RSL fluctuations

Superimposed patterns of shoreline shifts at different orders of cyclicity. third-orderreflects the true shift of the shoreline.

The higher orders of cyclicity reflect overall trends, at increasingly larger scales of observation.

Notethat the second-order maximum regressive surface at the end of cycle C is superimposed on the first-order maximum flooding surface (end of overall transgression I).


Systems tracts are contemporaneous, linked depositional environments (or depositional systems); they are the building blocks of sequences and different types of systems tracts represent different limbs of a RSL curve Falling-stage (forced regressive) systems tract (FSST) Lowstand systems tract (LST) Transgressive systems tract (TST) Highstand systems tract (HST)

The various systems tracts are characterized by their position within a sequence, by shallowing or deepening upward faciessuccessions, or by parasequence stacking patterns

Components of lowstand systems tract (LST)

A significant amount of finer-grained sediment starts to accumulate in the deep-water environment as mudflow deposits.

Two sequence stratigraphic surfaces form during base-level fall: (i) The subaerial unconformity, which gradually expands basinward as the shoreline

regresses; and (ii) The regressive surface of marine erosion (RSME) cut by waves in the lower

shoreface.

Depositional processes and products of the early falling stage systems tract

Most of the sand that accumulates during this stage is captured within detached and offlappingshoreline to upper shorefacesystems.

Late falling stage systems tract. The sediment mass balance changes in the favor of the deep-sea submarine fans, which capture most of the sand.

The subaerialunconformity keeps forming andexpanding basinward until the end of base level fall

.Once the shoreline falls below the shelf edge, the regressive surface of marine erosion stops forming as the sea floor gradient of the continental slope is steeper than what is

required by the shoreface profile to be in equilibrium with the wave energy.

Fluvial systems are likely to incise into the highstand prism but may only bypass the rest of the subaerially exposed shelf, unless the base level falls below the elevation of the

shelf edge.

The turbidity currents of the deep basin are dominantly of high density type, due to the massive amount of sediment supply, and hence they tend to be overloaded and

aggradational (sediment load > energy of the flow) along their entire course.

Amplitude extraction map along a seismic horizon, showing detached anddownstepping forced regressive shoreface deposits on the continental shelf.

The color code uses blue for sand and orange for shale.

Lowstandsystems tract: In contrast to the falling stage systems tract the sediment ofthis stage of early-rise normal regression is more evenly distributed between the fluvial, coastal, and deep-water systems

Sand is present in amalgamated fluvial channel fills, beach and delta front systems, as well as in submarine fans. The lowstand prism gradually expands landward via fluvial aggradation and onlap.

Aggradation on the continental shelf in fluvial to shallow marine environments reduces the amount of sediment supply to the deep basin, and hence the turbidity currents of this stage are dominantly of low-density type.

Depositional elements of a low-density turbidite leveed-channel system on a basinfloor

Levees are better developed along the outer channel bends, and their inner margins arecharacterized by the presence of scoop-shaped slump scars.

Transverse sections through the leveed channel

The 2D seismic lines indicate channel aggradation, as well as lateral migration with time.

Note that the sandy channel fill is characterized by higher amplitudeseismic reflections relative to the surrounding finer-grained facies of the overbank

environment. Levees are also built by finer-grained material relative to the channel fill.

The 2D seismic line shows the complex nature of the canyon fill, which recorded multiple stages of aggradation and erosion related to the activity of

gravity flows.

The arrow in the upper image shows the current direction of gravity flows.

Modern seafloor seismic imaging (Top)) and cross-sectional view (Bottom)of the Mississippi canyon (Gulf of Mexico)

Outcrop expression of lowstand fluvial systems

Shoreface

Amalgamated channel fill

Components of Transgressive systems tract (TST)

Early transgressivesystems tract

Most of the riverbornsediment is now trapped in fluvial,coastal and shallow marine systems

Wave ravinement processes erode the underlying normal regressive shelf-edge deltas and open shoreline systems, continuing to supply sand for the deepwater

turbidity flows. These turbidity flows tend to be of low-density type, similar to lowstandsystems tract

Estuaries are diagnostic for transgression, but retrograding or even progradingdeltas may also form in river mouth settings during the transgression of the open

shoreline, primarily as a function of degree of channel incision and sediment supply

Types of coastlines that may develop during base-level rise

Dictated by the balance between sedimentation rates and the rates of base levelrise

Aerial photograph

showing a river-dominated,

prograding delta in an overall

transgressivesetting

Canadian archipelago

High sed

iment sup

ply

Relative

high bas

e level

Late transgressivesystems tract

Most of the terrigenous sediment trapped in fluvial, estuarine, deltaic, open shoreline,and lower shoreface deposits

Additional sand is incorporated within shelf macroforms (sheets, ridges, ribbons) generated by storm surges and tidal currents.

Such shelf-sand deposits are generally associated with the transgressive systems tract, as the best conditions to accumulate and the highest preservation potential

The river mouth settings may become estuaries or deltas, depending on the balance between accommodation and sediment supply

Components of highstand systems tract (HST)

Highstand (late rise normal regression) systems tract. The deposits of this stage overlie and downlap the maximum flooding surface. The bulk of the

highstand prism includes fluvial, coastal and shoreface deposits. The shelf and deep marine environments receive mainly fine-grained hemipelagic and

pelagic sediments.

Aerial photograph of a Pleistocene highstand coastal prism Utah

The arrow points to localized fluvial incision, which is limited to the highstandprism.


Systems tracts are contemporaneous, linked depositional environments (or depositional systems); they are the building blocks of sequences and different types of systems tracts represent different limbs of a RSL curve Falling-stage (forced regressive) systems tract (FSST) Lowstand systems tract (LST) Transgressive systems tract (TST) Highstand systems tract (HST)

The various systems tracts are characterized by their position within a sequence, by shallowing or deepening upward faciessuccessions, or by parasequence stacking patterns

Outcrop examples of stacked parasequences.

Parasequences are prograding, coarsening-upward successions bounded by flooding surfaces.

Parasequence boundaries (i.e., flooding surfaces) mark events leading to abrupt increases in water depth (arrows).

Fall

Dominant types of gravity flows (1) cohesive debris flows (mudflows); (2) high-density turbidity currents and grainflows, forming proximal frontal splays (3) lower-density turbidity currents, forming leveed channels and distal frontal splays.


Maximum flooding surfaces form during the culmination of RSL rise, and maximum landward translation of the shoreline, and constitute the stratigraphic surface that separates the TST and HST

In the downdip realm (deep sea), where sedimentation rates can be very low during maximum flooding, condensed sections may develop

LSTs are separated from overlying TSTs by transgressivesurfaces; transgression is further characterized by coastal onlap

An alternative approach to sequence analysis uses genetic stratigraphic sequences that are bounded by maximum flooding surfaces

Incised meander belts formed during stages of base-level fall

A subsurfaceexample depicting a time slice through a 3D seismic volume

B interpretation of the features observed in

image A.

C modern example of an incised meanderbelt

Summary of criteria that may be used to differentiate between incised-valley fills and unincised or distributary channel fills.

Synthetic gamma ray logs illustrating the stratigraphic context of (I) incised-valley fills and (II) unincised channel fills

Incised-valley fills occupy an anomalous position in the

stratigraphic context, being genetically unrelated to the

juxtaposed facies

Unincised valley fills are genetically related to the juxtaposed and underlying facies


Maximum flooding surfaces form during the culmination of RSL rise, and maximum landward translation of the shoreline, and constitute the stratigraphic surface that separates the TST and HST

In the downdip realm (deep sea), where sedimentation rates can be very low during maximum flooding, condensed sections may develop

LSTs are separated from overlying TSTs by transgressivesurfaces; transgression is further characterized by coastal onlap

An alternative approach to sequence analysis uses genetic stratigraphic sequences that are bounded by maximum flooding surfaces


In a very general sense, RSL fall leads to reduced deposition and formation of sequence boundaries in updip areas, and increased deposition in downdip settings (e.g., submarine fans)

RSL rise leads to trapping of sediment in the updip areas (e.g., coastal plains with a littoral energy fence) and reduced transfer of sediment to the deep sea (hemipelagic deposition; condensed sections)


Seismic stratigraphy is based on the principle that seismic reflectors follow stratal patterns and approximate isochrons(time lines)

Reflection terminations provide the data used to identify sequence-stratigraphic surfaces, systems tracts, and their internal stacking patterns

Technological developments have been prolific: Vertical resolution improved to a few tens of meters Widespread use of 3D seismic

Seismic data should preferably always be interpreted in conjunction with well log or core data


A better understanding of stratigraphic sequences can be obtained by the construction of chronostratigraphic charts (Wheeler diagrams); these can subsequently be used to infer coastal-onlap curves

Variations in sediment supply can produce stratal patterns that are very similar to those formed by RSL change (except for forced regression); in addition, variations in sediment supply can cause stratigraphic surfaces at different locations to be out of phase

In principle, sequence-stratigraphic concepts could be applied with some modifications to sedimentary successions that are entirely controlled by climate change and/or tectonics (outside the realm of RSL control)

Dip-oriented cross-section through a hypothetical extensional basin. Locations A, B, and C are characterized by different subsidence rates,

A Sequence stratigraphic surface is not always synchronous

Diachroneity of surfaces

Changes in sea level, subsidence, and relative sea level during a period of time of 1.5 My; Incremental changes in time steps of 100,000 years. The curve of sea-level changes is the same for the three reference locations

Subsidence rates increase towards the basin: 0 m/105 yrs, 5 m/105 yrs and 10 m/105 yrs for location A, B, C.

Eustasy combined with subsidence allows for the calculation of the relative sea level change ( RSL) for each time step.

The cumulative relative sea level ( RSL) is calculated in the last column of the table. Key: * (x 105 yrs), # m/105 yrs, + m.

Subsidence, eustatic, and relative sea-level curves for the 1.5 My time interval .

Note that for location A where subsidence is zero, the sea-level curve coincideswith the relative sea-level curve.

For locations B and C the relative sea-level curves account for the combined effects of eustasy and subsidence.


A better understanding of stratigraphic sequences can be obtained by the construction of chronostratigraphic charts (Wheeler diagrams); these can subsequently be used to infer coastal-onlap curves

Variations in sediment supply can produce stratal patterns that are very similar to those formed by RSL change (except for forced regression); in addition, variations in sediment supply can cause stratigraphic surfaces at different locations to be out of phase

In principle, sequence-stratigraphic concepts could be applied with some modifications to sedimentary successions that are entirely controlled by climate change and/or tectonics (outside the realm of RSL control)


The global sea-level curve for the Mesozoic and Cenozoic (inferred from coastal-onlap curves) contains first, second, and third-order eustatic cycles that are supposed to be globally synchronous, but it is a highly questionable generalization Conceptual problems: spatially variable RSL change due to

differential isostatic and tectonic movements undermines the notion of a globally uniform control

Dating problems: correlation is primarily based on biostratigraphythat typically has a resolving power comparable to the period ofthird-order cycles

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Sequence Stratigraphy Concepts